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Discussion Starter #1
I recently got a '93 400E and like to use a battery tender and or charger sometimes to make sure the battery stays charges since I don't drive the car all the time. But, I noticed in the owner's manual it says to only charge the battery when it's disconnected from the car. Is this just being overly cautions? Any reason why I can't hook up a tender or charger while the battery is connected to the car?
 

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'95 E300 DIESEL, '91 600SEL, '92 600SEL
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Consider when that manual was written before the age of electronic pulse chargers.

Any NOCO or Optimate charger will do just fine. For a V8, the Optimate 6 or 7 Ampmatic is ideal and that is what I use for my W140s yearround. The Optimate continually monitors and periodically self tests the battery, and disconnects/reconnects itself electronically when it senses the battery is no longer fully charged. This avoids the classic problem of traditional battery tenders turning a battery stale.

 

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Battery chargers turn on and off as they maintain the power within a battery. Perhaps Mercedes was concerened about the potential damage that might happen to the sensitive electronics in the 400e. These cars were Mercedes' first substantial foray into digital electronics managing the engine and other systems. By today's standards, these 124s aren't sophisticated but that doesn't mean they weren't fragile. Your best solution would be to install a cutoff switch at the positive terminal of the battery and wire the charger to the positive terminal. This way, you can easily disconnect power to the vehicle and then charge the battery itself.

The only downside to this that I can think of is that any alarm that is in the car will be disabled when power is shut off. Additionally your radio might lose its "code" and presets if it is left disconnected for a long period of time. The condition of the internal battery in the Becker will determine how long it can hang in before you have to manually put this information back in.

Edit: I see Steve has come up with a better solution.
 

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Some battery tender/charger have a "repair" mode. Do NOT use this mode while the battery is connected to the car. It applies pulse charges up to 15 volts which can damage electronic modules.

My NOCO has this function and you have to hold the button down for a while to activate it (safety feature).
 

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Be wary of these tenders that claim they can de-sulfide batteries..."save" them. This is not something real science can replicate satisfactorily with tenders that "de-sulfide" batteries. No one is certain why they work of IF they will even work that way on any given battery.

So what I'm saying is; don't base your tender choices on this feature, but the other more worthwhile features listed above. And as always with Chinese made electronics; YMMV.

Kevin
 

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Discussion Starter #7
I have one more question.,...I will probably have to leave the trunk open to connect the battery tender, will the trunk light go off automatically after a certain period of time?
 

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No, you must remove the bulb. Otherwise you will have a constant draw on the battery.
 

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My NOCO has this function and you have to hold the button down for a while to activate it (safety feature).
That button is the single biggest reason why I retired my NOCO (Bosch branded) charger. Every time there is a power outage, the charger stays in standby mode but does not automatically resume after power comes back unless someone pushes the button again. Not acceptable for vehicles which are stored in a remote location. Hence the reason why I swapped over to Optimate for all my vehicles/bikes.

I don't believe in the desulfation feature that these new chargers feature, and have yet to successfully revive a single battery using that battery. The Optimate charger can detect if the battery is in the vehicle or not, and limits voltage and current depending on the condition.
 

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I read two white papers on de-sulfating batteries and while it CAN work, it's not reliable and repeatable. So to actually seek out a charger for that feature, or to pay more money for that feature is moot.

So....if you already have that feature, it's 'worth a shot' on a bad battery. But......certainly don't seek out tenders that balyhoo about this feature in their literature.

Kevin
 

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I have one more question.,...I will probably have to leave the trunk open to connect the battery tender, will the trunk light go off automatically after a certain period of time?
On some MBs with the plunger switch, you can pull the plunger out to shut off the light.
 

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Kevin,

I can say with confidence it doesn't work. I recently tried it again a few weeks ago on a Yuasa battery that had sat too long in a R129 without being recharged.

The charger goes through all the motions and ultimately tells me the cycle is complete (battery saved), BUT what my tester tells me with each desulfation cycle is that it builds up internal resistance. I suppose that is probably due to sulfation falling off the plates and building up at the bottom of the battery.

A battery simply goes bad with time, whether used or not regardless of type. The only battery that can kept indefinitely is the kind that is dry charged at the factory, but the acid has NOT been added yet. As soon as you remove the airtight foil on the battery prior to adding the acid, the lifetime clock starts ticking.
 

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That button is the single biggest reason why I retired my NOCO (Bosch branded) charger. Every time there is a power outage, the charger stays in standby mode but does not automatically resume after power comes back unless someone pushes the button again.
My NOCO G3500 resumes charging after power outage. Just checked. Repair mode requires button to be held down to be reactivated.
 

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Nice, but mine doesn't. Also the proprietary NOCO quick connect plug is a royal PITA. That is another reason why I changed brands since Optimate uses the conventional SAE connector used by many brands. That is the second biggest weakness with the NOCO for me.
2617951
 

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One concern with connecting a charger while in the car is also that you can potentially boil the electrolyte and acid will come out the vent holes damaging whatever is around. In my humble opinion that was the concern of the Owners Manual...

Sent from my moto g(7) power using Tapatalk
 

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One concern with connecting a charger while in the car is also that you can potentially boil the electrolyte and acid will come out the vent holes damaging whatever is around. In my humble opinion that was the concern of the Owners Manual...

Sent from my moto g(7) power using Tapatalk
That may be true with high amperage chargers. I never had that problem with tenders or chargers set on low amperage. I always do badly discharged batteries out of the vehicle.
 

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One concern with connecting a charger while in the car is also that you can potentially boil the electrolyte and acid will come out the vent holes damaging whatever is around.
In general, you should never charge at more than 10% of the rated capacity. Example: if you have a 100Ah battery like the group49 size batteries used in the W124, you should charge at no more than 10Amp charge rate.

Thankfully, with the current generation electronic pulse chargers it will determine the optimum charge rate to prevent boiling the acid or causing other battery damage.
 

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For the last 20 year for just storage (through the long New England winters) I have always just used the el-cheapo ($9.99 each) Harbor Freight float chargers on all of my cars that get stored and they have always worked perfectly with no ill effects. The current output from these things is inherently low so you can't actually attempt to recharge a battery (so the battery must be fully charged when you start), but at the same time, the low available current output limits the potential for any damage. There are three of them at work right now in the 10 degree night air keeping things ready for spring! I do have a modern electronic pulse type charger for use when I really to charge a battery as opposed to just cover the standby current draw during storage.
 

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I have a MC that used to be charged long term (I'm a fair weather rider). I bought a MC charger at Walmart that everyone said was great. I lost two batteries to that thing over the yrs and it went in the trash can. MC batteries are crazy expensive....analogous to anything boat related. Next time I try one of these electronic marvels, albeit mostly made in China.

Kevin
 

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One concern with connecting a charger while in the car is also that you can potentially boil the electrolyte and acid will come out the vent holes damaging whatever is around. In my humble opinion that was the concern of the Owners Manual...

Sent from my moto g(7) power using Tapatalk
Some group 49 batteries actually have a left or right (your choice) vent fitting where you can add a rubber hose and direct the fumes any place you want. I choose the rain drain closest to the battery.

Kevin
 
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